In recent years, in order to improve the visibility of image display devices, filling of the gap between an image display panel such as LCD, PDP or EL and a protective panel or a touch panel member disposed on the front side (viewing side) thereof has been carried out with a pressure-sensitive adhesive sheet, adhesive or the like, thereby suppressing the diffused reflections that occur when there is no filling, i.e., the diffused reflections at the air/layer interface, of an incoming beam or an outgoing light from the displayed image.
For instance, a display device is described in Patent Reference 1, in which the gap between the LCD (liquid crystal panel display) and a touch panel is filled with a transparent resin (adhesive), allowing the reflection of light to be reduced between the touch panel and the liquid crystal display panel.
A problem has been pointed out, that when the gap between an image display panel and a protective panel or a touch panel member is filled with a transparent resin (adhesive) in this way, if a defect or the like exists at the interface between the image display panel and the transparent resin (adhesive) or at the interface between the protective panel or the like and the transparent resin (adhesive), a gas build-up occurs with this defect as the origin, by exposure under a high temperature and high humidity environment or by exposure to a sudden change in temperature, provoking bubbling or peeling.
Consequently, a variety of studies have been carried out in order to solve such problems.
For instance, production methods for display devices with an optical function member built-in are proposed in Patent References 2 and 3, which involves photo-curing (or heat-curing) an adhesive composition beforehand and producing an adhesive composition that is in a partially crosslinked state, then followed by bonding together an optical function member and a display portion via this adhesive composition, and thereafter heat-curing (or photo-curing) the adhesive composition by heat treatment (or light irradiation).
A method described in Patent Reference 4 involves including a high molecular weight acrylic polymer within a pressure-sensitive adhesive composition thereby increasing cohesive strength, imparting resistance to bubbling, and suppressing peeling.
In addition, the present applicants have proposed in Patent Reference 5 or the like, a novel transparent adhesive sheet that can be adhered without generating residual air bubbles, even if there were an unevenness of on the order of 5 μm to 20 μm on the adhered surface, and furthermore can be adhered without bubbling under a high temperature environment of, for instance, on the order of 80° C. even if the adherend were a material that generates outgas such as plastic.